Numerical precision for probabilities / simplex variables

Hello,

In my model, I have a term of the following form: \frac{1-e^x}{1-k e^x}, where k>0 and x can range from -inf to inf.

In my case, such a term appears in several closed form expressions describing the simple birth and death process (a.k.a. Kendall process, a Markov birth death process). For example, the probability of the process going extinct by time t is P_{extinct}[t;\lambda,\mu]=\frac{\mu}{\lambda} \left( \frac{e^{(\lambda-\mu)t}-1}{e^{(\lambda-\mu)t}-\frac{\mu}{\lambda}} \right).

I am running into issues with numerical precision when the x in \frac{1-e^x}{1-k e^x} approaches -inf. (In my case, x=(\lambda-\mu)t and t is my data in minutes. Some values of t reach ~80 minutes, which makes x take on values like -60.) The term approaches 1, but I cannot have the term equal 1 exactly.

In one case I’ve debugged, I can’t have the term equal to 1 because I am using it as a probability p_extinct in a simplex: simplex theta = [prob_extinct, prob_event2, prob_event3]. The components of theta must sum up to 1. When I calculate prob_extinct=1 and prob_event2 = 1e-80, for example, then attempt prob_event3 = 1 - prob_extinct - prob_event2, then I get prob_event3 < 0, which leads to a NaN if I work with log probabilities. Alternatively, if I compute prob_event3 separately, Stan throws an error because the sum of the terms of theta is >1. My Stan model fails to initialize when run because the initial 100 parameter guesses (e.g. for \lambda and \mu) always lead to theta containing NaN’s.

Computing \log(\frac{1-e^x}{1-k e^x}) as log1m_exp(x) - log1m_exp( x + log(k) )  helps with precision of the numerator and denominator separately, but doesn’t get around the fact that I’m subtracting two similar numbers from each other. Is there another clever way to compute \frac{1-e^x}{1-k e^x} using the composed functions available in Stan?

Or is the problem elsewhere, perhaps in that I am not using simplex variables in their intended usage? Should I have an if statement like if(p_extinct == 1){ return(1 - machineeps()) } to make sure I never return exactly 1 for p_extinct?
In my model, the simplex is used like this: mycountdata ~ multinomial( theta ).

It seems to me that you can special-case the likelihood when prob_extinct is large, so that you avoid doing the simplex thing for large enough prob_extinct and just expect a deterministic zero (i.e. don’t increment the target at all, provided that the data are zero, and increment by negative infinity otherwise) when that happens.

If that runs into edge cases or something, then you could also try declaring prob_event2 and prob_event3 as a 2-simplex, and then scale them by one minus prob_extinct.